1. Field of the Invention
This invention relates to a control apparatus for a hydraulically operated vehicular transmission which is mounted on automobiles and other vehicles.
2. Description of the Related Art
A control apparatus for a hydraulically operated vehicular transmission has a shift valve unit with controls the supply of hydraulic oil to and the discharging of hydraulic oil form hydraulic pressure coupling elements for the speed change steps of a transmission, so as to selectively establish each of the speed change steps. The shift valve unit is switched according to the predetermined speed change characteristic which is determined with engine throttle opening degrees and vehicle speeds as parameters, to thereby perform an automatic speed changing operation.
Roughly stated, the control apparatuses are divided into ones of electronic control type, and ones of pure hydraulic pressure control type. In the control apparatus of electronic control type, an electromagnetic valve which is controlled by a controller which receives signals from a throttle sensor and a vehicle speed sensor, is used to switch the shift valve unit. The control apparatus of pure hydraulic pressure control type includes: throttle hydraulic pressure signal outputting means for outputting a throttle hydraulic pressure signal according to a throttle opening degree; and vehicle speed hydraulic pressure signal outputting means for outputting a vehicle speed hydraulic pressure signal according to a vehicle speed. In the control apparatus, the shift valve unit is switched according to the throttle hydraulic pressure signal and the vehicle speed hydraulic pressure signal so that the speed change gear stage on the low speed side is established as the throttle hydraulic pressure signal becomes high while the speed change gear stage on the high speed side is established as the vehicle speed hydraulic pressure signal becomes high. Recently, the control apparatus of electronic control type has been popularly employed; however, because of the manufacturing cost, the control apparatus of pure hydraulic pressure control type has been employed, too.
On the other hand, the speed change is carried out in the order of a low speed step, a middle speed step, and a high speed step; however, if the throttle opening degree is abruptly changed, sometimes the speed change is carried out from the low speed step directly to the high speed step skipping the middle speed step. For instance, when the throttle opening degree is abruptly increased, the gear is shifted down from the high speed step directly to the low speed step with skipping from the middle speed step. In addition, when the throttle opening degree is abruptly decreased, the gear is shifted up from the low speed step directly to the high speed step with skipping the middle speed step. If this skipping speed change is carried out, then the shock is liable to occur. Hence, it is desirable that the shift valve unit is switched so that, even if there is occurred a state that the skipping speed change is effected by the abrupt change of the throttle opening degree, the speed change is effected through the middle speed step.
In order to meed this requirement, Japanese Patent Examined Publication No. 27785/1993 has disclosed the following control apparatus of pure hydraulic pressure control type. In the control apparatus there are provided first and second control valves. The first control valve is disposed on a throttle hydraulic pressure signal input oil path coupled to the low-speed--middle-speed shift valve of the shift valve unit and shuts off the above-mentioned input oil path until the hydraulic pressure of a high speed step hydraulic pressure coupling element is decreased to a predetermined value. The second control valve is disposed on a vehicle speed hydraulic pressure signal input oil path coupled to the middle-speed--high-speed shift valve and shut off the above-mentioned input oil path until the hydraulic pressure of a low speed step hydraulic pressure coupling element is decreased to a predetermined value. With the control apparatus, even when the throttle opening degree is abruptly increased, the middle-speed--high-speed shift valve is switched, so that hydraulic oil is discharged from a high speed step hydraulic pressure coupling element, and until the hydraulic pressure is decreased to a predetermined value, the switching of the low-speed--middle-speed shift valve over to the low speed side is prevented by the action of the first control valve. Accordingly, the gear is smoothly shifted down from the high speed step through the middle speed step to the low speed step. On the other hand, even when the throttle opening degree is abruptly decreased, the low-speed--middle-speed shift valve is switched, so that hydraulic oil is discharged from a low speed step hydraulic pressure coupling element, and until the hydraulic pressure is decreased to a predetermined value, the switching of the middle-speed--high-speed shift valve over to the high speed side is prevented by the action of the second control valve. Accordingly, the gear is smoothly shifted up from the low speed step through the middle speed step to the high speed step.
The above-described related control apparatus needs the first control valve for preventing the skipping down shift, and the second control valve for preventing the skipping up shift. Therefore, it is unavoidably complicated in structure.